It depends mostly on your role. Swiftcast is universally useful for all casters, although healers place a particular premium on it for Raise and dps purposes. Black Mages get it automatically, while summoners can speed up their rotation by applying a dot or placing a shadow flare in the initial pull.

DPS casters really want to level archer for Raging Strikes (lv.4) and Quelling Strikes (lv.34). These will increase critical hit rate and reduce enmity, especially useful for black mages who decide to double- or triple-flare in the first 20 seconds of a fight, or summoners who want to crit all their aoes and bane/fester without pulling from the tank in the iniital pull.